Boudinage Classification

Journal of Structural Geology 26 (2004) 739–763 www.elsevier.com/locate/jsg

Boudinage classiﬁcation: end-member boudin types and modiﬁed boudin structuresq

Ben D. Goscombea,*, Cees W. Passchierb, Martin Handa

aContinental Evolution Research Group, Department of Geology and Geophysics, Adelaide University, Adelaide, S.A. 5005, Australia bInstitut fuer Geowissenschaften, Johannes Gutenberg Universitaet, Becherweg 21, Mainz, Germany

Received 28 March 2002; received in revised form 25 August 2003; accepted 30 August 2003

Abstract In monoclinic shear zones, there are only three ways a layer can be boudinaged, leading to three kinematic classes of boudinage. These are (1) symmetrically without slip on the inter-boudin surface (no-slip boudinage), and two classes with asymmetrical slip on the inter-boudin surface: slip being either (2) synthetic (S-slip boudinage) or (3) antithetic (A-slip boudinage) with respect to bulk shear sense. In S-slip boudinage, the boudins rotate antithetically, and in antithetic slip boudinage they rotate synthetically with respect to shear sense. We have investigated the geometry of 2100 natural boudins from a wide variety of geological contexts worldwide. Five end-member boudin block geometries that are easily distinguished in the field encompass the entire range of natural boudins. These five end-member boudin block geometries are characterized and named drawn, torn, domino, gash and shearband boudins. Groups of these are shown to operate almost exclusively by only one kinematic class; drawn and torn boudins extend by no-slip, domino and gash boudins form by A-slip and shearband boudins develop by S-slip boudinage. In addition to boudin block geometry, full classification must also consider boudin train obliquity with respect to the fabric attractor and material layeredness of the boudinaged rock mass. Modified or complex boudin structures fall into two categories: sequential boudins experienced a sequence of different boudin block geometry components during progressive boudinage (i.e. continued stretch), whereas reworked boudins were modified by subsequent deformational episodes (folded, sheared and shortened types). Correct classification of boudins and recognition of their modification are the crucial first stages of interpretation of natural boudin structures, necessary to employing them as indicators of shear sense, flow regime and/or extension axes in terranes otherwise devoid of stretching lineations. q 2003 Elsevier Ltd. All rights reserved.

Keywords: Boudin; Boudinage; Shear zones; Modiﬁed boudin; Kinematic indicator

1. Introduction Simpson and Schmid, 1983; Hanmer, 1984, 1986; Van der Molen, 1985; Ramsay and Huber, 1987; Goldstein, 1988; Boudinage is the disruption of layers, bodies or foliation Lacassin, 1988; DePaor et al., 1991; Hanmer and Passchier, planes within a rock mass in response to bulk extension 1991; Swanson, 1992, 1999). Nevertheless, there has been along the enveloping surface. Boudin structures were first no attempt to erect a classification scheme for the full range described by Ramsay (1881) and named by Lohest (1909). of natural boudin structures. We propose a simple The huge variety that occur in nature have been the subject classification scheme that accounts for all simple unmodi- of considerable study (Wilson, 1961; Ramsay, 1967; fied boudin geometries that have been described and further, Etchecopar, 1974, 1977; Hobbs et al., 1976; Lloyd and relate these end-member boudin geometries in a hierarchical Ferguson, 1981; Lloyd et al., 1982; Blumenfeld, 1983; scheme that considers their relationship to the three kinematic classes by which boudinage occurs (Table 1; q Extended datasets, Appendices B–C, can be found in the online version Goscombe and Passchier, 2003). These kinematic classes of this paper are: (1) symmetrical boudins, which do not experience slip * Corresponding author. Correspondence address: Geological Survey of Western Australia, P.O. Box 1664, Kalgoorlie, W.A. 6433, Australia. on the inter-boudin surface, forming by no-slip boudinage, E-mail address: [email protected] (B.D. Goscombe). and two asymmetrical classes with slip on the inter-boudin

Table 1 Classiﬁcation of boudinage of monoclinic symmetry

Below the thick shaded line are real boudin structures observable in rocks and above it, their correlating kinematic class of formation. Only boudin types that have not been modified are included here. All natural boudin structures can be classified in terms of the five end-member boudin block geometries that are highlighted by shading. These can be further sub-divided and the common examples are given (see also Fig. 3). surface: (2) synthetic slip boudinage (S-slip) and (3) 2. The boudin dataset antithetic slip boudinage (A-slip) with respect to bulk shear sense. Our dataset contains over 2100 boudins and collates the In this paper, we classify natural boudin structures, both most important geometric parameters and features describ- simple and modified, according to boudin block geometry, ing boudin structures illustrated in Fig. 1 and defined in boudin train geometry and nature of the boudinaged Appendix A. A summary of the dataset is available in material. This analysis is based on an extensive dataset, Table 2. Data were obtained primarily from the authors’ collected by the authors and collated from the literature, of field studies in a wide range of geological environments; quantitative data and qualitative features that describe including non-coaxial shear environments (Zambezi Belt in boudin structures from a wide range of geological Zimbabwe; Ugab Terrane, Damara Orogen and Kaoko Belt environments, spanning different flow regimes, rheological in Namibia; and the Himalayas in Nepal), pure shear contrasts, metamorphic grade and strain. The boudin environments (Adelaidean Fold Belt in South Australia) and structures involved have been analysed in two integrated transtensional environments (oceanic crust on Macquarie ways. First, natural groupings have been delineated based Island). These data are augmented by measurement of on boudin block geometries, boudin train geometries and photographs of boudins in true profile (normal to the boudin the nature of the boudinaged material that can be recognized long axis) from the authors’ collections and from the in the field. Second, we have related geometric groups to literature (Wilson, 1961; Ramsay, 1967; Weiss, 1972; bulk flow and kinematic groups wherever possible. We Hobbs et al., 1976; Lloyd and Ferguson, 1981; Borradaile largely restrict our study to boudin structures of monoclinic et al., 1982; Lloyd et al., 1982; Ramsay and Huber, 1983, symmetry, considering the 2D-geometry in the profile plane 1987; Sengupta, 1983; Simpson and Schmid, 1983; Bos- normal to the long axis of the boudin. Where necessary, 3D worth, 1984; Fry, 1984; Hanmer, 1984, 1986; Mullenax and effects are included. We use the results of this analysis to Gray, 1984; Van der Molen, 1985; Davis, 1987; Goldstein, establish criteria that define simple end-member boudin 1988; Malavielle and Lacassin, 1988; Hanmer, 1989; types and subsequently recognize these components within Laubach et al., 1989; Passchier et al., 1990; DePaor et al., complex modified boudin structures. The latter can be 1991; Goscombe, 1991, 1992; Hanmer and Passchier, 1991; either: (1) reworked boudin structures modified by sub- Swanson, 1992; Carreras and Druguet, 1994; Yamamoto, sequent deformation unrelated to boudinage or (2) sequen- 1994; Daniel et al., 1996; Lisle, 1996; Conti and Funedda, tial boudin structures that contain multiple end-member 1998; Kraus and Williams, 1998; Roig et al., 1998). Data boudin type components and formed by progressive from photographs comprise 60% of the total dataset. The boudinage. photographs do not permit the measurement of parameters B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 741

3. Boudin structural elements

3.1. General parameters

Boudins are complex and highly variable 3D structures and so to be uniquely described requires a large number of quantifiable dimensional and angular parameters relating the structural elements (Fig. 1; Appendix A). No consistent nomenclature for boudin structural elements or geometric parameters has been adopted in the literature, even for the simplest system of symmetric boudinage (Jones, 1959; Wilson, 1961; Hobbs et al., 1976; Penge, 1976; Lloyd et al., 1982). Thus, a suite of nomenclature for structural elements and geometric parameters that uniquely describes all possible boudin structures is adopted in this study (Fig. 1) and defined in detail in Appendix A. A description of the nature of boudinage involves first of all the overall nature of the boudinaged element. The nature or degree of layeredness of the boudinaged material forms a continuous spectrum that can be classified into the following. (1) Object boudinage of a competent object of limited dimensional extent such as a mineral grain. (2) Single-layer boudinage of a competent layer in a less competent host (Ghosh and Sengupta, 1999). (3) Multiple- layer boudinage of a packet of thin competent layers (Ghosh and Sengupta, 1999). A variation is ‘composite boudins’ composed of boudinaged sub-layers within a boudinaged Fig. 1. Nomenclature and symbols of boudin structural elements and packet of layers, giving nested boudins of different scale geometric parameters. (a) Asymmetric types (example is domino boudins). (Ghosh and Sengupta, 1999). (4) Foliation boudinage (b) Sigmoidal-gash boudin (left), forked-gash boudin (right). (c) Symmetric (Lacassin, 1988; Mandal and Karmakar, 1989; Ghosh and types; torn boudins (top) and drawn boudins (bottom). See Appendix A for Sengupta, 1999) of a foliated rock devoid of, or irrespective full definitions. Layer in (b) is for reference only, typically gash boudins are in foliation boudinage. Shear sense is dextral in all cases. of, layers of differing competence (Fig. 2). An alternative way of describing this class is in the ratio C/G of the width of more competent or boudinaged units against the grain- out of the profile plane, such as those referenced to the size; if this ratio is less than 10, boudinage does not occur in boudin long axis, but all other important parameters can be distinct layers and is known as foliation boudinage. measured. Boudinage is the disruption of layers, bodies or foliation All scales of boudinage are considered from metre- planes within a rock mass resulting in boudin blocks (simply scale to mineral grain-scale (i.e. Simpson and Schmid, called boudins). For single-layer boudinage, it is easy to 1983; Lister and Snoke, 1984; Simpson, 1984; Goscombe, define a boudin exterior (Sb), the disrupted fragments of the 1991, 1992; Yamamoto, 1994; Goscombe and Everard, original bounding surfaces of the boudinaged body. For 2000; Goscombe, unpublished data). It was found that multiple-layer and foliation boudinage, Sb can be defined as nearly all groups of boudins, with their characteristic the surface connecting the boudin edges or tips of boudin parameters, are developed on both the mesoscopic and the faces (Fig. 1). In addition, an enveloping surface (Se) can mineral grain-scale. Parameters measured from both usually be defined for a train of boudins (Fig. 1). Se is outcrop and photographs of the same structure indicate commonly, but not always, parallel to the penetrative that there is no systematic skewing of the data with either foliation (Sp) in the host rock to the boudins. The orientation measurement source. Consequently, data from photo- of the boudin long axis, boudin edge or neck zone can be graphs (i.e. from the literature) are considered equally described as a linear feature, Lb (Fig. 1). Lb is by definition accurate as those measured in the field. Absolute errors on normal to the extension axis (Le) of the boudin structure measurements can only be estimated, but comparison (Appendix A). An angle d can be defined between Lb and between outcrop and photographic data offer limits. The the dominant stretching lineation (Lp) in the host rock to the average absolute difference between measurements from boudins. This angle d is usually orthogonal, in which case outcrop and photograph of the same structure is on the boudins and adjacent structures have a monoclinic 3D order of ^4.58 for angular parameters and ^5–10% for geometry. The vast majority of boudins investigated have linear parameters. monoclinic symmetry and in such cases parameters 742

Table 2 Mean values of parameters and diagnostic features of the ﬁve end-member boudin block geometries in foliation-parallel boudin trains. Drawn boudins have been further sub-divided into necked and tapering boudins

Asymmetric boudins Symmetric boudins Domino boudins Gash boudins Shearband boudins Torn boudins Tapering boudins Necked boudins

Number of boudins 479 79 340 499 361 94

Boudin shape and nature

Inter-boudin surface (Sib) Sharp, straight Sharp, sigmoidal or forked Ductile, sigmoidal/straight Sharp, straight or concave None None Sib –Se relationship Sib terminates as Se Sib discordant to Se Sib terminates as Se Sib discordant to Se No Sib, stretch of Se No Sib, stretch of Se Boudin shape Angular rhomb Angular rhomb Sigma lens or rhomb Angular block Convex lens Pinch and swell ..Gsob ta./Junlo tutrlGooy2 20)739–763 (2004) 26 Geology Structural of Journal / al. et Goscombe B.D. Associated kinkbands None None Rare None None None Inter-boudin zone ﬁll Host (68%), vein (32%) Vein (75%), host (25%) Host (96%), vein (4%) Vein (74%), host (26%) Host (95%), vein (5%) Host (99%), vein (1%) Foliation boudinage 35% 95% 0% 8% 0% 0% Typical boudinaged layera Psammite, calc-silicate Psammite Quartz-vein in schists Calc-silicate, psammite Maﬁc, aplite Aplite Preferred metamorphic gradea Low-grade Low-grade High-grade Low-grade High-grade High-grade

Boudinage with respect to tectonic transport

Slip on Sib A-slip (98%) A-slip (68%), no-slip (30%) S-slip (100%) None (99%) None (100%) None (100%) Block rotation sense Synthetic (98%) Synthetic (64%), none (32%) Antithetic (100%) None (97%) None (100%) None (100%) Kinematic class A-slip (98%) A-slip (100%) S-slip (100%) No-slip (96%) No-slip (94%) No-slip (100%) Atypical geometry for kinematics 3% 0% 0% NA NA NA

Flanking fold on Sib 42% 62% 0% NA NA NA Synthetic drag on Sib 13% 0% 98% NA NA NA No deﬂection on Sib 45% 38% 2% 100% NA NA Vergence by Sib inclination Forward vergent (100%) forward Vergent (91%) Backward vergent (100%) Forward vergent (56%) NA NA Second episode of boudinageb 10% 5% 11% 8% 35% 8%

Angular parameters (mean) a—relative block rotation 188 108 168 None None None u—block shape 728 818 398 858 NA NA 0 u —inclination of Sib wrt Se 548 708 248 858 NA NA b d—Lb wrt Lstretching 718 738 778 718 748 718 b Distribution of Lb wrt Lstretching Symmetrical around Lstretching 86% clockwise of Lstretching Symmetrical around Lstretching Symmetrical around Lstretching Symmetrical around Lstretching 63% clockwise of Lstretching

Dimensional parameters (mean) L/W—aspect ratio 1.99 1.83 3.57 2.90 4.07 2.59 D/W—normalized displacement 0.49 0.12 2.20 0.00 NA NA

% with displacement on Sib 100% 85% 100% 4% NA NA N/L—Sib dilation (if present) 0.17 0.05 0.02 0.41 NA NA % with dilation across Sib 29% 78% 2% 100% NA NA M/L—normalized extension 0.32 0.07 0.66 0.41 0.77 0.29

Stretch—extension of Se 126% 104% 160% 141% 177% 129% % with isolated boudins 17% 0% 70% ,98% 93% 0% Flanking fold half wavelength/W 0.65 0.31 1.21 0.14 NA NA

Data sourced from all investigation areas and literature. Symbols and nomenclature deﬁned in Appendix A. a Normalized for both the total number of readings of each boudin block geometry category and total number in each rock-type or metamorphic grade category. b Data from Kaoko Belt only. Brackets indicate proportion of data in the indicated category. B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 743

Fig. 2. Full classiﬁcation scheme, outlining the three aspects that describe the full geometry of any boudin train. Diagrams illustrate a representative example from each type. Shading indicates where transitions exist between groups. All diagrammatic examples illustrating boudin train obliquity have domino boudin block geometry and the fabric attractor (FA) is assumed for simplicity to be parallel to the pervasive foliation (Sp) where present. Note: the foliation-parallel boudin train formed by A-slip boudinage and all foliation-oblique boudin train examples formed by S-slip boudinage (cf. Goscombe and Passchier, 2003). Shear sense is dextral in all cases.

measured in the profile plane normal to Lb are sufficient to common scenario of brittle host inflow is accommodated by fully describe boudin geometry. In some cases, however, a network of minor faults in the inter-boudin zone, aligned d , 908 and the 3D geometry of the boudins and adjacent at a high angle to layering. True dilation and vein infill is host rock is triclinic. A full 3D-description of the structure is typically hydrothermal material (such as quartz, micas and necessary in such cases. carbonate) at lower amphibolite facies and lower grade In all cases, boudin blocks are separated by either a conditions and pegmatite and partial melt material at upper discrete surface called the boudin face, or an extensional amphibolite and granulite facies. gap known as the inter-boudin zone or neck zone (Fig. 1). For any boudin type with boudin faces or inter-boudin The inter-boudin zone is a complex feature into which there zones, an inter-boudin surface (Sib) can be defined. In is material transfer by host inflow, vein infill or combi- asymmetric boudin structures without dilation, Sib is an nations of both (Fig. 3). Host inflow is typically ductile, inclined, discrete surface that coincides with the boudin resulting in deflection of the enveloping surface and face, along which boudins were laterally displaced and formation of half folds (‘scar folds’, Hobbs et al., 1976), which arcs into parallelism with the enveloping surface (Se) with characteristic peaked and cuspate shapes that are best at the boudin edge (Figs. 1 and 4). In symmetric and known in their association with boudin structures. The less asymmetric boudins with dilation, Sib is not a discrete 744 B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763

Fig. 3. Schematic drawings of variations in the five end-member boudin block geometries (in centre of diagram) and relationship to both shear strain (outwards from centre of page) and relative proportion of host inflow (i.e. scar folds) versus vein infill in the inter-boudin zone (down page). An increasing ratio of host inflow over vein infill indicates increasing flattening strain associated with layer extension. Asymmetric (LHS) versus symmetric (RHS) grouping of boudin type illustrates that shearband types are essentially asymmetric drawn boudins and domino types are essentially asymmetric torn boudins. Shear sense is dextral in all asymmetric cases, as shown. surface developed in the rock but is the imaginary ‘skeletal’ (stretch); boudin block isolation (M0) and layer extension median plane in the inter-boudin zone (Fig. 1b). In blocky (M). These latter parameters are related to the basic ones by torn boudins this is typically sub-parallel to the boudin face the equations: (Fig. 1c). In drawn boudins no Sib as defined above is a ¼ Sb ^ Se ¼ arctanðDsinuðL þ DcosuÞÞ ð1Þ developed, thus an arbitrary ‘Sib‘ defined as the median 0 plane connecting boudin terminations is used in these cases u ¼ Se ^ Sib ¼ u 2 a ð2Þ (Fig. 4c; Appendix A). In contrast to faults or shear zones, M ¼ Dcosu0 þ NðcosasinuÞð3Þ the Sib is confined to the boudinaged layer and so differs from ‘pseudo-boudinage’ or ‘structural slicing’ (Bosworth, M0 ¼ M 2 Wðcosu0cosð9082uÞÞ ð4Þ 1984), where layer disruption is due to penetrative faults and stretch ¼ðM þ LcosaÞ=L ð5Þ shear zones that extend beyond the disrupted layer. Boudinage ranges in scale from mineral grain- to metre- 3.2. Parameters quantifying boudin block geometry scale and thus absolute dimensional parameters are of little interest in defining boudin geometry. Consequently, all In order to compare boudins, quantitative parameters dimensional parameters, such as displacement on Sib (D), 0 must be defined that capture the essential features of these dilation across Sib (N), boudin isolation (M ) and layer structures (Fig. 1; Appendix A). It is not always necessary to extension (M), are normalized with respect to width (W)or describe the geometry of boudins with great accuracy, and length (L) of the boudin block as outlined in Table 2. The in that case, a few essential and most diagnostic parameters, angle u and three independent normalized parameters such as those listed in Table 2, are sufficient for quantitative suffice to completely describe skeletal boudin geometry. characterization. For a complete description of skeletal Boudin block shape is given as a description of the outer boudin geometry in a section normal to Lb, the following surface of the boudin in profile. This is typically parameters suffice (Fig. 1): L, the length of individual characterized by groupings with descriptive names that boudin blocks; W, boudin thickness or width; u, the angle encompass a circumscribed range in values of quantitative between Sb and Sib; D, the displacement of Sb along Sib; and and qualitative parameters. The most important parameters N, the width of the inter-boudin gap measured normal to Sib. are aspect ratio (L/W), shape of the boudin face, shape of the Some additional parameters can be useful in describing boudin exterior and degree of asymmetry (u)(Figs. 1 and 4). boudin structures, although they are dependent on the five Shape of the boudin face in profile ranges from terminating basic elements mentioned above. These additional par- at a point (tapering or lenticular boudins), convex-face (such 0 ameters are: the inclination of Sib (u ), the relative block as ‘sausage’ boudins), straight-face (such as blocky rotation (a; the extension of the enveloping surface boudins), concave to extremely concave and folded against B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 745

Fig. 4. (a)–(c) Deﬁnition of the four parameters that comprehensively deﬁne boudin block shape. u ¼ Symmetry of boudin block, L/W ¼ aspect ratio,

Bb ¼ curvature of boudin exterior and Bf ¼ curvature of boudin face. (d) Range of symmetric boudin block shapes and nomenclature of the most common examples. itself as in the case of ‘fishmouth’ or ‘false-isocline’ boudins rhomb shapes to asymmetric tapering shapes with low u. (Figs. 3 and 4). Shape of the boudin exterior (Sb) can be Degree of angularity of the boudin edge may be angular concave (such as ‘bone-type’ boudins), convex (such as such as blocky boudins or curved such as sausage, sigma- ‘barrel’ boudins) or parallel to each other (parallelograms shaped and lenticular lenses. such as blocky boudins; Figs. 3 and 4). Boudin exteriors In some cases, a more precise geometric description of may be parallel but not planar only in cases of reworked boudin blocks is needed. Boudin shape can be simply boudin structures that have been sheared or folded quantified by ratios that define the degree of convex or (described later). Boudin block asymmetry is defined by concave curvature of the boudin faces and boudin exteriors the angle between Sib and Sb (u), ranging from symmetric (Fig. 4; Appendix A). The ratio describing the curve of the boudins with high u (such as blocky boudins) through boudin exterior (Bb) is defined by the maximum normal 746 B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 deviation of the boudin exterior out from (þve) or into the scale and different site, on the margin of the trace of boudin interior (2ve), from the straight line connecting the foliation-oblique boudin trains (Fig. 5a; Hanmer and boudin edges, divided by the length of this line (i.e. L). In Passchier, 1991; Ramsay and Lisle, 2000). the same way, the curve of the boudin face (Bf) is defined by 4. The sigmoidal trace of Sib has S- or Z-shape defining a the maximum normal deviation of the boudin face out from spiral vergence that is either synthetic or antithetic to (þve) or into the boudin interior (2ve), from the straight bulk shear sense (Fig. 1b). This vergence is analogous to line connecting the edges of the boudin face, divided by the that in sigmoidal tension gashes. length of this line (Fig. 4). For asymmetric boudins, the same can be applied, and the combination of L/W and u,Bb Comparison of natural boudin geometry and bulk shear and Bf is in most cases sufficient to describe the shape of sense indicate that the characteristic geometry of different boudin blocks for practical purposes (Fig. 4). end-member asymmetric boudin types can indeed be linked to bulk shear sense and thus kinematic class of boudinage in many cases, as explained in detail in Goscombe and 4. Boudinage asymmetry Passchier (2003). Here, we focus on boudinage classification, which includes but is not restricted to the record of Many boudins are asymmetric and are potential shear kinematics. sense indicators (Goscombe and Passchier, 2003). Several parameters define the asymmetry vergence of sets of boudin blocks in a boudin train, as follows: 5. End-member boudin block geometry types

Boudins in our dataset can be rationalized into a minimal

1. The direction of inclination of Sib defined by the angle set of five distinct end-member boudin block geometries, (u) between Sib and Sb is either in the opposite direction which can be readily recognized in the field (Table 1; Figs. 2 to shear sense and named forward-vergent or inclined in and 3). These are domino, shearband, gash, drawn and torn the same direction as shear sense and named backward- boudins. These can be grouped into symmetric types (drawn vergent. Swanson (1992, 1999) used the terms ‘forward- and torn boudins) and two asymmetric boudin groups: tilted‘ or ‘forward-rotated’ and ‘backward-tilted’ or domino types (domino and gash boudins) and shearband ‘backward-rotated’ to indicate vergence by the direction types (Table 1). The five end-member boudin block of hypothetical tilting of an originally orthogonal Sib with geometries are characterized in detail below, geometric respect to shear sense. The Sib did not necessarily rotate parameter fields are presented in Figs. 6–8, diagnostic into the inclined orientation (Goscombe and Passchier, features and average parameters are summarized in Table 2, 2003) and so the terms ‘tilting’ and ‘rotated’ are typical forms are represented in Figs. 1–3 and natural misleading and should be discontinued. examples in Fig. 9. The essential elements required to define 2. Lateral displacement of the boudin exterior (Sb) on the all boudin types are presented here; further details for Sib, also known as slip on the inter-boudin surface, is asymmetric boudins are provided by Goscombe and expressed by a value D. Its sense can be either synthetic Passchier (2003). (S-slip) or antithetic (A-slip) with respect to bulk shear sense (Figs. 2 and 3). Directly associated with this slip is 5.1. Domino boudins the direction of relative rotation (a)ofSb with respect to the enveloping surface, Se (block rotation). In S-slip Domino boudin blocks are asymmetric, usually short, boudinage, block rotation is antithetic and in A-slip stubby rhomb shapes with angular edges. In most cases boudinage, block rotation is synthetic with respect to Sib is a discrete sharp surface (planar domino boudins) or bulk shear sense. in 29% of investigated cases is a set of parallel terminal 3. Foliation or layering within the boudin may be deflected faces either side of an inter-boudin zone filled with vein in a narrow zone parallel to Sib; this can be in the same material or host rock (dilational domino boudins; Fig. 9a; (synthetic drag) or opposite (antithetic drag) direction as Table 1). Dilational domino boudins have straight Sib; the displacement sense on Sib (Fig. 5b and c). Such those with sigmoidal Sib trace are classified as gash behaviour has been described as ‘flanking structures’ by boudins (Table 1). Sib is at a high angle to Sb (u typically Passchier (2001). Structures with synthetic drag are 55–908; Fig. 7) and together with typically low lateral known as flanking shear bands and those with antithetic displacement (D) along Sib, implies low extension of the drag as flanking folds (Grasemann and Stuwe, 2001; enveloping surface (stretch averages only 126%; Table 2). Passchier, 2001). Flanking folds on Sib contribute an In all domino boudins, inclination of Sib is in the opposite additional component of block rotation that is synthetic direction to bulk shear sense, defining forward-vergent with whole block rotation (Figs. 5b and 6). Flanking structures, and almost all correspond to A-slip boudinage folds on Sib are not to be confused with the flanking folds (Table 1). Apparent ‘drag’ on Sib, where present, is that formed by a similar mechanism, but on a different antithetic to slip on Sib, resulting in development of B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 747

Fig. 5. Types of ﬂanking structures associated with boudins: nomenclature after Passchier (2001). (a) Flanking folds on the margins of foliation-oblique boudin trains. (b) Flanking folds (antithetic ‘drag’) on the boudin face (Sib) of domino boudins. (c) Flanking shear-band (synthetic ‘drag’) on the boudin face of shearband boudins. (d) Combination of synthetic ‘drag’ (kink-bend termination) and antithetic ‘drag’ (ﬂanking fold) on the boudin face of gash boudins.

ﬂanking folds (Hudleston, 1989; Passchier, 2001; Figs. 1b sliding’ (Ramsay and Huber, 1987), ‘sheared stack of cards and 5). Antithetic ﬂanking folds on Sib are diagnostic of model’ (Etchecopar, 1974, 1977; Simpson and Schmid, domino-type boudins, including gash boudins, with high 1983), ‘type I asymmetric pull aparts’ (Hanmer, 1986), L/W ratio, and do not occur in other boudin types or ‘turf’ or ‘asymmetric blocky boudins’ (Hanmer, 1984) and domino boudins with low aspect ratios (Fig. 6). ‘extensional fracture boudinage’ or ‘forward-rotating Domino boudins of the geometry described above have orthogonal vein geometry’ (Swanson, 1992, 1999). The been variously called: ‘tuilage/tiling structures’ (Blumen- term domino has been used informally by many workers feld, 1983; Hanmer and Passchier, 1991), ‘book shelf (i.e. Hanmer and Passchier, 1991), is widely understood and 748 B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763

Fig. 6. Relationship between relative block rotation (a) and aspect ratio (L/W), by boudin block geometry type. Range of the two types of block rotation— whole-block rotation and rotation in the boudin face zone by ﬂanking folds—are indicated with respect to L/W.

Fig. 7. Relationship between boudin block shape (u) and relative block rotation (a) for each boudin block geometry type. Large symbols represent foliation- parallel boudin trains and small symbols represent foliation-oblique boudin trains. best encapsulates their geometry in one word and so has characteristics that they can be grouped separately from been adopted for this classiﬁcation of boudins. domino boudins (cf. Goscombe and Passchier, 2003). All are dilational and have a sigmoidal Sib trace that is either a 5.2. Gash boudins smooth continuous curve or of angular geometry (Fig. 1b); thus we have named these gash boudins (Table 1). Smooth Some domino-type boudins that formed almost exclu- forms resemble sigmoidal tension gashes (Fig. 1b). Angular sively by foliation boudinage have such a distinct set of forms are more complex, and typically comprise a straight

Fig. 8. Relationship between dimensional ratios, with ﬁelds for each boudin block geometry type outlined. (a) Normalized displacement on Sib (D/W) versus boudin aspect ratio (L/W). (b) Relative thickness of either the layer (drawn boudins) or vein inﬁll (torn boudins) in the inter-boudin zone relative to the boudin.

(c) Normalized dilation across Sib (N/L) versus normalized layer extension (M/L). Not all drawn boudins have been represented, because in all cases N/L ¼ 0 and the distribution is similar to that of shearband boudins. B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 749 750 B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763

Fig. 9. Examples of typical unmodified end-member boudin block geometries. (a) Dilational domino boudins in carbonate from Nanga Parbat Massif in Pakistan. (b) Forked-gash boudin in sheared feldspathic quartzite from Kaoko Belt, Namibia. (c) Tapering boudins of pegmatite in amphibolite-grade gneisses from Huckitta Creek, Arunta Block. (d) Multiple-layer boudinage of inter-layered quartzite and carbonate, with drawn boudin geometries, from the Kaoko Belt in Namibia. (e) Foliation boudinage of sheared quartzo-feldspathic gneisses from the Kaoko Belt, Namibia. Torn boudins with extreme concave face, commonly called fishmouth boudins. (f) Foliation-oblique boudin train of tapering boudins of discordant quartz vein in carbonate matrix, Ugab Zone, Namibia. Note flanking fold (arrow) on the margin of the boudin train, due to rotation of the train to lower angles (C) with the pervasive foliation, during progressive deformation. Bulk shear sense is dextral in all asymmetric cases (some images have been flipped about a vertical axis to facilitate comparison). central section and forked or inclined crack terminations respectively. We favour sigmoidal-gash boudins and (Figs. 1b and 9b), which have been described by Swanson forked-gash boudins as names for these geometries. (1992) as ‘swordtail’ and ‘fishmouth’ terminations. Swan- Gash boudins have some other special characteristics. son (1992) called the two gash boudin types ‘sigmoidal The central portion of Sib is nearly orthogonal (u averages boudin partings’ and ‘reoriented extension fractures’, 818) to the boudin exterior (Sb). Aspect ratios of boudin B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 751 blocks are extremely low (Table 2) and unlike all other and asymmetric shearband boudins (Fig. 2a). In addition, boudin types, the extent (Q; Fig. 1b) of the boudin structure shearband boudins have been variously named ‘type II along Lb is clearly finite, averaging only 90% of the boudin asymmetric boudins’ (Goldstein, 1988), ‘sigmoidal boudi- length, L. Dilation and associated vein infill are common but nage’ (Ramsay, 1967), ‘shear-fracture boudinage’ (Swanson, narrow (Fig. 8c) and displacement is low (Fig. 8a), resulting 1992), ‘backward-rotated shear-band boudins’ (Swanson, in very low extension of the enveloping surface (stretch 1999), ‘surf asymmetric pull-apart structures’ (Hanmer, 1984), averaging only 104%). Slip on Sib is usually antithetic to ‘asymmetric extensional structures’ (Lacassin, 1988) and bulk shear sense and Sib inclination is forward-vergent ‘asymmetric pull-apart structures’: type 2A without discrete (Table 2). Flanking folds are common in the central portion Sib and type 2B with discrete Sib (Hanmer, 1986). We have of Sib and synthetic drag is only evident at the tips of Sib adopted the name shearband boudins because their geome- (Fig. 5d), resulting in the ‘kink-bend’ terminations try so closely resembles that of shear-band cleavages described by Swanson (1992) and Grasemann and Stuwe (Simpson, 1984; Lister and Snoke, 1984) and in a single (2001). Spiral vergence defined by the sigmoidal trace of Sib word best invokes an image of their form. is synthetic to bulk shear sense. 5.4. Drawn boudins 5.3. Shearband boudins Both shearband and domino boudins grade into Shearband boudins are asymmetric with rounded rhomb symmetric boudin structures. These symmetric boudins to tapering lens shapes, typically with relatively high aspect can be grouped into those that experienced apparent ratios (Fig. 8a). The obtuse edge of the boudin is often ‘ductile’ stretch of the layer (named drawn boudins) and rounded and the acute edge is drawn into a tapering wing by those where the layer was dissected by high-angle, drag on Sib. Dilation across Sib and associated vein infill sharp, apparently ‘brittle’ planes of failure (named torn almost never occurs (Fig. 8c). Sib is typically a thin ductile boudins; Table 1). Drawn boudins are symmetric with shear zone with associated ductile grain refinement and no block rotation apparent and no inter-boudin surface grain-shape fabric. In some cases, Sib constitutes a wider (Sib); layer extension was accommodated by differential ductile zone displacing adjacent boudins with the appear- stretch of both the layer and host. Drawn boudins vary ance of asymmetric drawn boudins. Sib is straight to continuously in both the degree of boudin separation and curviplanar and at a low angle to Sb,withu typically less degree of thinning of the interconnecting neck zone. This than 608 (Fig. 7). Lateral displacement on Sib is the highest spectrum is arbitrarily subdivided (Table 1; Fig. 3)into of all boudin types (Fig. 8a). Consequently, extension of Se necked boudins with boudin ‘blocks’ still connected by a is high (Fig. 8c), with stretch averaging 160% and neck zone and tapering boudins that are either completely commonly (69% of investigated cases) results in complete isolated (Fig. 9c), or tenuously connected by particularly isolation of adjacent boudin blocks (Table 2). All shearband long (M/L @ 0.3) and thin (W0W , 0.1) necks (Fig. 8b). boudins form by S-slip boudinage (Table 1) and are Necked boudins are also called pinch-and-swell struc- backward-vergent. Drag on Sib is almost always evident and tures in the literature (Ramsay, 1967; Penge, 1976; Lloyd synthetic to slip (Table 2)—a diagnostic feature that is et al., 1982; Van der Molen, 1985). Necked boudin ‘blocks’ responsible for the tapering sigma shapes of the boudin blocks. have moderate aspect ratios (L/W averaging 2.6) and are Shearband boudins of foliation boudinage type are typically of classical boudin shape with bi-convex exteriors probably more common than represented in our dataset. and less commonly parallel exteriors with a thinned inter- Such structures would be expressed as sigma-shaped lenses boudin neck zone (W0W averaging 0.38; Figs. 8b and 9d) of schist within schist host, best described as ‘foliation fish’ associated with host inflow. Vein emplacement (Hanmer, 1986) and akin to mica fish (Lister and Snoke, accompanied thinning in only 1.4% of investigated cases. 1984), forming a continuum between what is true boudinage Extension of Se is relatively low with stretch averaging only and what is due to a penetrative C or C0 shear-band 129% (Table 2). cleavage. The geometry of ‘pseudo-boudinage’ structures Tapering boudin blocks have curved shapes with bi- formed by penetrative C0 shear-band cleavages has been convex exteriors and terminations that are typically drawn investigated (n ¼ 15), and these differ in no way from into pointed terminations (Fig. 9c) but can also be rounded, shearband boudins except for the lateral extent of the C0- resulting in symmetric boudins with lens/lozenge, or less surface (equivalent to Sib). Mineral grain-scale shearband commonly, sausage shapes (Fig. 9d). Hence the alternative boudins do occur; mica fish (Lister and Snoke, 1984) may names ‘lenticular’ boudins (Lloyd et al., 1982)and be a common example with forms that mimic the typical ‘stretched layer’ (Lacassin, 1988). Vein infill in the inter- sigma shape of mesoscopic shearband boudins. boudin zone is almost entirely absent, with layer extension Shearband boudins of the geometry described above have being accommodated entirely by inflow of the host rock. been called ‘asymmetric pinch-and-swell structures’ Aspect ratios (L/W averaging 4.07), extension of the (Hanmer, 1984, 1986; Hanmer and Passchier, 1991), drawing enveloping surface (stretch averaging 177%, with a attention to the continuum between symmetric drawn boudins maximum of 643% in the Kaoko Belt) and isolation of 752 B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 boudin blocks (M0L averaging 0.69) are all the highest of Furthermore, there is minimal overlap or transition between any boudin type (Table 2). Tapering boudins with high the five end-member boudin block geometries, which on the aspect ratios also have high layer extension (M) and boudin whole maintain distinct and readily recognizable groups in isolation (M0), suggesting that at high strain the boudin the field (Figs. 6–8). Transitions between the different block is being flattened concomitant with boudin separation. groups define a one-dimensional, or linear, spectrum with transitions between drawn and shearband, shearband and 5.5. Torn boudins domino, domino and sigmoidal-gash, sigmoidal-gash and torn boudins, but apparently not between symmetric boudin Torn boudins have sharp inter-boudin surfaces (Sib), types, drawn and torn boudins (Figs. 2, 7 and 8). equivalent to the boudin face, across which there was There is a transitional continuum between torn and significant dilation, which is equivalent to layer extension domino boudins. Both u and u0 in domino boudins vary 0 (M/L ¼ N/L $ M L, which averages 0.41; Figs. 1 and 8c). continuously up to typical torn boudin values of 908 (Fig. 7). Sib is at a high angle and typically orthogonal to the Both torn and domino boudins typically develop sharp, 0 boudinaged layer (u ø u averages 858), resulting in planar inter-boudin surfaces; across which dilation is angular, block-shaped boudins also called ‘rectangular’ or diagnostic of all torn boudins and occurs in 29% of domino ‘extension fracture’ boudins (Lloyd and Ferguson, 1981; boudins as the subset called dilational domino boudins. For Lloyd et al., 1982) and ‘rectilinear’ boudins (DePaor et al., most parameters, torn boudins grade progressively into 1991). Torn boudins can be conveniently sub-divided into sigmoidal-gash boudins and then domino boudins (Figs. straight-face and concave-face boudins (Table 1; Fig. 3). 6–8), thus the latter two can be considered progressively The vast majority of boudin block shapes are angular and more asymmetric torn boudins. A small proportion of torn blocky with parallel exteriors. However, continued stretch boudins (4.5%) show incipient asymmetry with very small of the layer often results in ‘burring over’ of the boudin displacements and/or block rotations of ,58, all of which edges, concave boudin faces, and bi-convex exteriors, have domino-type vergences and formed by A-slip resulting in what has been called ‘barrel-shaped’ boudins boudinage (Table 2). Vergence defined by inclination of (Lloyd and Ferguson, 1981). The degree of boudin face S shows a weak predominance in torn boudins (56%) to concave curvature varies continuously (Fig. 3)from ib being forward-vergent like in domino-type geometries. At straight-face to fishmouth boudins (DePaor et al., 1991; the other end of the domino boudin spectrum, all geometric Swanson, 1992; Fig. 9e) which are also called ‘fish-head’ parameters vary continuously with shearband boudins, such (Ghosh and Sengupta, 1999) or ‘extreme barrel-shaped’ as decreasing a, u, u0 and N and increasing D (Figs. 6–8). In boudins (Lloyd and Ferguson, 1981; Lloyd et al., 1982), and the field of overlap (i.e. 508,u , 608) distinguishing ultimately to where the face is folded against itself to between these two boudin types is unreliable and other resemble false isoclinal folds. Fishmouth boudins are best qualitative features must be investigated such as the nature developed in laminated rock types such as carbonates and by foliation boudinage (Fig. 9e). The boudin face is convex of Sib, fringe folds, drag folds and independent shear sense in less than 5% of investigated torn boudins. indicators (Goscombe and Passchier, 2003). Boudin exteriors (Fig. 1) typically remain parallel or There is a transitional continuum in the degree of slightly convex, but are also rarely concave in the case of asymmetry of ‘ductile’ boudinage types from drawn bone-type boudins (Malavielle and Lacassin, 1988; Swan- boudins to shearband boudins (Fig. 2a). Shearband son, 1992) that have undergone continued flattening, after boudins can be considered asymmetric drawn boudins vein infill of the inter-boudin zone, resulting in W0W . 1 and vice versa. The neck zone in both boudin types is a (Fig. 3). The inter-boudin zone of torn boudins is typically ductile shear mobile zone, a symmetric coaxial shear filled by vein material (in 74% of investigated cases) zone in drawn boudins and asymmetric non-coaxial shear containing quartz, K-feldspar, carbonate or micas at zone in shearband boudins. At very low angles of u and 0 greenschist and amphibolite facies and by partial melt at u , the inter-boudin surface of shearband boudins upper-amphibolite to granulite grades. The enveloping becomes indistinguishable from a highly attenuated foliation is symmetrically drawn into the inter-boudin neck zone connecting boudin terminations in the case zone to varying degrees, resulting in scar folds (Hobbs of drawn boudins, and block rotation is barely percep- et al., 1976). These are of half wavelength, have peaked tible (Fig. 7). The angular parameters (‘u’ and a) in this (Ramsay, 1967), cuspate or box-fold profiles (Lisle, 1996), transitional field are all ! 108 and cannot be measured which are fold forms generally developed only in associ- accurately; drawn boudins are characterized by ation with boudinage. ‘u’ ¼ a ¼ 08. The inter-boudin surface in some shearband boudins is not a discrete surface but a wide ductile 5.6. Transitions shear mobile zone displacing adjacent boudins. In these cases, the boudin train has the appearance of asymmetric Typically, only one boudin type is developed within a drawn boudins, which have been described as ‘asym- single boudin train; no mixed trains were recognized. metric pinch-and-swell structures’ (Hanmer, 1984, 1986; B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 753

Hanmer and Passchier, 1991) and ‘type 2A asymmetric the orientation of the fabric attractor, especially if boudins pull-apart structures’ without discrete Sib (Hanmer, 1986). have a complex history, but it is nevertheless important to be aware of the importance of this feature, especially if boudins are to be employed as shear sense indicators or flow regime 6. Boudin train obliquity indicators (Passchier and Druguet, 2002; Goscombe and Passchier, 2003). The geometry of boudin structures, especially of Three foliation-oblique boudin train scenarios exist. asymmetric boudins, is not only dependent on material Unlike foliation-parallel boudin trains, foliation-oblique parameters, orientation of Sib during the initial strain boudin trains do not necessarily form in association with a increment, bulk flow geometry, metamorphic conditions penetrative foliation in the rock mass, but are necessarily and strain but also on the initial orientation of the oblique to the fabric attractor (Fig. 2c). ‘Fabric-associated’ boudinaged element with respect to extension eigenvectors foliation-oblique boudin trains result where an originally of flow in the rock, also known as fabric attractors, such as oblique layer (i.e. discordant vein or dyke) is boudinaged contained in the flow plane of simple shear (e.g. Passchier, during development of the penetrative foliation in the host 1997). The angle between the fabric attractor in the profile rock (Fig. 9f). ‘Massive’ foliation-oblique boudin trains plane and the boudin train enveloping surface (Se)is form in discordant layers without development of a labelled C in this paper (Figs. 2 and 5a). Boudin trains can penetrative foliation or any other feature approximating be divided into two distinct classes based on boudin train the fabric attractor (i.e. vein or dyke in an unfoliated matrix obliquity (Fig. 2c). Boudin trains lying at a low angle to the such as in a granite; Hanmer and Passchier, 1991). ‘Short- fabric attractor, for which the final value of Cf attained at limb’ foliation-oblique boudin trains form where the fabric the end of deformation is imperceptible in the field and less attractor can be inferred to be at an angle to layering and than a value chosen to be 108, are classified as foliation- foliation in the rock mass, but with no new fabric developed parallel boudin trains (Fig. 2c). All others, where the angle during boudinage (i.e. boudinage of the over-turned limb C 8 f exceeds 10 , are classified as foliation-oblique boudin during asymmetric folding). In the last two scenarios, a new trains (Fig. 2c). The difference in kinematic behaviour foliation defining the fabric attractor during boudinage is not between these two classes is discussed in Goscombe and necessarily developed. Consequently, these are difficult to Passchier (2003). recognize as foliation-oblique boudin trains and alternative Boudinage of a material line must necessarily occur in clues to the obliquity of the fabric attractor must be sought. the extensional flow field, but the angle of the boudin train Foliation-parallel boudin trains typically form in bedding to the fabric attractor is important for their rotational during development of a bedding-parallel penetrative behaviour. Rotation of boudins with respect to the fabric foliation, and are by far the most numerous type. They attractors depends on boudin aspect ratio and orientation, comprise 96% of investigated boudin trains in the Kaoko whereas the boudin train acts as a material line and rotates Belt, and this high proportion is probably typical of similar towards the attractor in all cases. This can set up relatively complex block rotation of boudins. Boudins can, for schistose deformation belts that underwent transpressional example, form in general flow parallel to the flow strain. Four of the five end-member boudin geometry types eigenvector so that individual boudins may rotate forward, are developed in both foliation-parallel and foliation- but the boudin train is stationary with respect to the fabric oblique boudin trains, with the exception that gash boudins attractor, and domino boudins are the result. If the same apparently only form in foliation-parallel boudin trains. The boudin train is inclined at a high angle (such as 608) to the range in, and average values of, geometric parameters in attractor, the material line will rotate faster than the boudins, each of the end-member boudin geometries do not differ and shearband boudins can form. In general, boudin blocks significantly between foliation-parallel and foliation- rotate at a slower rate than the boudin train, and because oblique boudin trains (Figs. 6–8). Thus the five end- boudinage will only occur where the boudin train rotates member boudin block geometries remain independent of, forward into the extensional flow field, foliation-oblique and equally distinctive across, the full range of boudin train boudin trains typically operate by S-slip boudinage regard- obliquity. Nevertheless, we have noted that the end-member less of the shape of the developed boudin block (Fig. 2c; boudin block geometries are not universally well developed Goscombe and Passchier, 2003). It is therefore important to in foliation-oblique boudin trains in all the terranes try to determine the possible orientations of boudin trains investigated in this study. This appears to be strongly with respect to the fabric attractor. Typically the fabric controlled by the flow regime experienced during boudinage attractor is approximately represented in the rock mass by a (Goscombe, unpublished data). For example, foliation- penetrative fabric (Sp and Lp), but only if the local oblique domino boudin trains are only recognized in deformation history is relatively simple. However, a terranes that experienced pure shear flattening, whereas penetrative foliation may not be developed, or where foliation-oblique shearband boudin trains dominate in present, may not represent the fabric attractor associated transpressional terranes that are otherwise devoid of with boudinage. In practice, it can be difficult to reconstruct foliation-oblique domino boudin trains. 754 B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763

Fig. 10. Classification of reworked boudins that have been modified by a second deformation. Schematic drawings illustrating the most common types recognized in the areas investigated and from the literature. Boudins that have been modified by further stretch (i.e. progressive boudinage) are a special case called sequential boudins that are illustrated in Fig. 11. Symbols as in Fig. 11. Shear sense is dextral in all cases.

7. Modified boudin structures folded boudins (Fig. 10). Reworked boudin structures form where boudins have been subjected to a second phase of 7.1. Modified boudin structures: theory deformation separated from the first by a significant period of time, i.e. polyphase, non-congruent structures. (2) A significant subset of investigated boudin structures Sequential boudin structures experienced continued exten- show evidence for continued deformation of the boudin sion; this can imply that deformation during subsequent blocks, after the boudins have separated (Weiss, 1972; episodes was similar to the original condition that caused Ramsay and Huber, 1983; Hanmer and Passchier, 1991; the boudinage, i.e. progressive congruent structures (Fig. Carreras and Druguet, 1994). Modified boudins can be 11). Critical to the interpretation of sequential boudin classified into two distinct groups (Figs. 10 and 11). (1) structures is recognizing successive components of boudi- Reworked boudin structures experienced shortening sub- nage of different geometric type. Separation of these sequent to formation; those shortened in coaxial flow are components is helped by formation of a time marker feature called shortened boudins, in non-coaxial flow are called such as vein material, or a change in external conditions sheared boudins and where the boudin train as a whole has such as metamorphic grade, strain or strain rate, between the been folded, either in coaxial or non-coaxial flow, are called component events. Non-coaxial or coaxial flow conditions B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 755

Fig. 11. Schematic drawings of the most common sequential boudins recognized in the areas investigated. Sequential boudin structures are the cumulative result of two or more boudin block geometry components formed during progressive boudinage. Shear sense is dextral in all cases. do influence further extension of boudins, but mainly in the facilitate recognition of modified boudin structures. Some degree of asymmetry of the structures, which can be modified boudins are straightforward to recognize, but weakened or strengthened in this way. others may closely resemble simple single-phase boudins. In the discussion of boudin modification it is important to In all cases, crucial information is stored in both the inter- realize that not all combinations of shortening and extension boudin zone and any enveloping foliations present along the of layers are possible in a flowing continuum. In any type of boudin exterior. In reworked boudin structures, this includes monoclinic non-coaxial flow with real eigenvectors, i.e. all shortening or folding of inter-boudin vein infill, or solution flow types between simple shear and pure shear, material or crenulation cleavage development in the inter-boudin lines can only rotate from the shortening into the extension zone or over the boudin exterior (Fig. 10). Similarly, in field. This means that during continuous deformation, a sequential boudin structures deformation is typically layer may be first shortened and possibly folded, and then partitioned into the inter-boudin zone, which is modified boudinaged, but NOT first boudinaged and then folded. For into more complex forms (Fig. 11). Note that some folding this reason, if shortened boudins are found, this is evidence types can develop as part of the normal boudinage process if for either an unusual flow type without real eigenvectors part of the fabric is rotated into the shortening field of local (rotating flows, like in an eddy), or that deformation is flow (i.e. kinkbands associated with shearband boudins). polyphase, with a change in the orientation of principal However, such folding will indicate shortening at high angle shortening directions between periods of deformation to the boudin envelope, not parallel to it. (Passchier, 1997). In practice, it may not be possible or useful to determine 7.2. Reworked boudin structures: overprinting under exactly what combination of deformation was involved in different external conditions the formation of a modified boudin structure. However, in all cases it is useful to know that such overprints exist, since A significant proportion of the boudins noted in the they indicate a change in the nature of deformation in an literature (Weiss, 1972; Ramsay and Huber, 1983; Mala- area. The main purpose of the following descriptions is to vielle and Lacassin, 1988; Hanmer and Passchier, 1991; 756 B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763

Swanson, 1992; Carreras and Druguet, 1994) and in the oblique boudin trains of the earlier formed inter-boudin vein areas investigated, have been modified to varying degrees material (Swanson, 1999). by later deformation. In most cases, reworked boudins are Layer-parallel shear of asymmetric boudin types can easily recognized as structures with disharmonic folding change boudin shape by rotation of Sib into either steeper or (i.e. shortening) of foliations in the enveloping surface (Fig. shallower inclinations and by internal deformation of the 10). Alternatively, boudin blocks may be deformed by a boudin blocks (Fig. 10). In high strain, high metamorphic second boudinage event with extension axis at a high angle grade ductile terranes, layer-parallel shear can result in the to the first, such as chocolate tablet boudins. Examples of two asymmetric boudin types being oppositely modified and extensional reworking of boudin structures are rare and hard the geometry of the two converging towards each other. For to distinguish from complex boudin structures that formed example, layer-parallel shear rotates Sib into lower incli- in a single event of high flattening strain (Casey et al., 1983; nations (i.e. u and u0 decrease) in domino boudins, resulting Ghosh, 1988; Aerden, 1991) or in triclinic flow. in what we call ‘shearband-like’ boudins. Conversely, Sib steepens in shearband boudins resulting in ‘domino-like’ 7.2.1. Folded boudin structures boudins (Fig. 10). Consequently, in high shear-strain, high- Modified boudins are typically, though not always, the grade terranes, domino and shearband boudins may be result of overprinting subsequent to boudinage by a second, incorrectly identified and caution must be employed when pervasive deformation event. Obvious examples of using these as shear sense indicators (Goscombe and reworked boudin structures are folded boudin trains (Fig. Passchier, 2003). All these examples of sheared boudin 10)(Ramberg, 1952; Sengupta, 1983). Where aspect ratios structures can potentially look like a simple end-member are high, individual boudin blocks may be folded without boudin type and be almost impossible to recognize as folding of the boudin train (Sengupta, 1983). If the boudins reworked boudin structures that experienced polyphase were wide apart, shortening can stack them like shingles histories. The complex history recorded by the final into ‘rooftile’ structures (Sengupta, 1983; Passchier et al., reworked boudin structure is best indicated by disharmonic 1990), and drawn boudins can develop into ‘ramp-folded’ folding of the enveloping surface (Fig. 10) and folding of structures (Fig. 10; Passchier et al., 1990; Price and internal layering within boudin blocks (Fig. 12d). Cosgrove, 1990). Asymmetric ramp-folded tapering boudins are common in the Kaoko Belt and Damara Orogen in 7.2.3. Shortened boudin structures Namibia, and the Himalayan investigation regions. In all of Shortening by coaxial flow, i.e. symmetric shortening of these regions, Lb is parallel to fold axes and the reworked a layer, results in the most spectacular changes in shape of structure apparently formed in one progressive shear event, boudins. If they do not deform much internally, torn boudins suggesting a rotating flow scenario (Passchier et al., 1990) can be shortened by compressing the inter-boudin zone, where boudin trains rotated from the extensional into the leading to folding of the enveloping foliation that had been shortening field. Structures with apparent asymmetric ramp- drawn into the gap (Fig. 12e). In extreme cases, it can lead to folded forms can also form by attenuation of the overturned extrusion of foliated matrix material or vein infill out of the limb of a fold without earlier boudinage (Swanson, 1999). inter-boudin zone. In the case of drawn boudins, the thin inter-boudin neck can be folded or affected by pressure 7.2.2. Sheared boudin structures solution seams. If folded, the thinnest portion of the neck Boudins may also be reworked by non-coaxial flow, not will show the smallest wavelength of folding as predicted by necessarily with shortening or extension; e.g. when they are buckling theory (Ramsay, 1967). If the boudins deform parallel to the flow plane of simple shear or another fabric internally, barrel-shaped or blocky boudins may obtain attractor. Layer-parallel shear of symmetric boudins pro- markedly rounded bi-convex exteriors with high Bb ratios, duces asymmetric structures similar to simple asymmetric resulting in a curious oval shape with a barrelling effect that boudin types, such as torn boudins that are sheared into is more extreme than anything developed in normal boudins domino-type or shearband-type geometries. If boudins do (Fig. 12f). Barrelling such as this is easily recognized where not deform much internally, slip on the inter-boudin surface boudin width is significantly greater than the relict can modify torn boudins into shapes resembling domino undeformed inter-boudin vein material (Fig. 12e and f). boudins. If the boudins do deform internally, shearband Alternatively, shortening of boudins with either parallel or boudin shapes can result from either drawn or torn boudins. bi-concave exteriors, such as bone-type boudins, will result Torn boudins with inter-boudin vein material result in what in ‘trapezoidal boudins’ if initial boudin block aspect ratios we call ‘asymmetric vein boudins’ that superficially were low (Sengupta, 1983; Fig. 10). Symmetric shortening resemble simple dilational domino boudins (Figs. 10 and of asymmetric boudins can lead to reverse faulting 12d). These have been called ‘asymmetric bone-type reactivation of Sib, and if the boudins are deforming boudins’ and have been shown to result from layer-parallel internally, into less asymmetric shapes that may be difficult shear rotating the inter-boudin veins into a consistently to recognize as having been reworked. In all cases, inclined array (Malavielle and Lacassin, 1988; Swanson, foliations and lineations that mantle the boudin train will 1992). Extreme layer-parallel shear may result in foliation- be shortened and can show the effects of this shortening by B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 757

Fig. 12. Examples of modified boudins. Sequential boudin structures ((a)–(c)) are special cases of modified boudins that are formed by progressive stretching resulting in temporally sequential components of different boudin block geometries. (a) Torn boudin with classic barrel boudin geometry of concave face and convex exterior, in mafic gneiss from the Kaoko Belt, Namibia. Progressive boudinage resulted in drawn boudinage of the inter-boudin zone vein infill resulting in an overall tapering boudin geometry. (b) Psammite layer with straight-face torn boudins and quartz-vein infill in the inter-boudin zone, from the Gander Zone in Newfoundland. Progressive boudinage and a change in competence hierarchy resulted in extreme flattening of the original boudin blocks that were drawn without modification of the more competent inter-boudin vein infill. (c) Original domino boudins widely separated by drawn boudinage at higher ductility. Example is in high metamorphic grade quartzo-feldspathic gneisses from the Kaoko Belt in Namibia. Reworked boudin structures (d)–(f). (d) Straight-face torn boudins in a psammite layer from the Ugab Zone, Namibia, have been modified by sinistral foliation-parallel shear. Quartz veins in the inter- boudin zones have been rotated into a consistently inclined array and boudin blocks have been folded in sympathy with the asymmetric tightening of scar folds in the host. (e) Sequential boudin structure from the Ugab Zone, Namibia, that involved progressive boudinage of carbonate layers. Straight-face torn boudins with vein infill were subsequently drawn into tapering boudin geometries. The sequential boudin structures were later modified by shortening along the length of the boudin train, producing thickening and folding of the boudin blocks. (f) Mafic concave-face torn boudins with quartz-vein infill in the inter-boudin zone, from Mount Isa in Australia. The boudin train was later modified by roughly layer-parallel shortening, resulting in thickening of the boudin blocks, producing extreme convex curvature of the exterior. The inter-boudin vein infill provides an approximate indication of the original layer width. 758 B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 the development of solution seams, crenulation cleavage, extension of a symmetric form (Appendix B). Possibly, this small folds or kinks. Crucially, the scar folds in the documents either a progression to lower strain rate or inter-boudin zone differ in shortened boudin trains by prograde metamorphism, and thus more ductile defor- having greatly enhanced amplitudes and tight, pinched mation. For example, bow-tie vein (DePaor et al., 1991) and hinges (Fig. 12f). bone-type boudins (Price and Cosgrove, 1990; Swanson, 1992; Fig. 12b) are sequential boudins with an early torn 7.3. Sequential boudin structures: overprinting under boudin and vein infill component, followed by drawn similar external conditions boudinage of either the inter-boudin vein (Hossein, 1970)or boudin block resulting in these two respective types (Fig. Sequential boudin structures are complex boudin struc- 11). Bone-type boudins develop where the vein material is tures that evolved through progressive deformation at stronger than the boudin material, resulting in subsequent similar conditions to those in the initial stages of boudinage. extension being partitioned into the boudin block, which is That is, they form by continuing stretch of the boudinaged stretched. Eight percent of torn boudins in the Kaoko Belt material and document the ontogenesisof the structure from are bow-tie vein boudins (Figs. 3 and 12a); these structures initiation to an ‘adult’ boudin form. Simple end-member have been noted elsewhere by previous workers (DePaor boudin block geometries (Fig. 2a) themselves constitute the et al., 1991; Ramsay and Lisle, 2000). In these structures, simplest category of sequential boudin structures, because the W0W ratio of the stretched inter-boudin vein averages they are not instantaneously formed. Examples of such 0.54 and is intermediate between torn boudins (W0W ¼ 1) gradual modification of boudin shape during one phase of and necked boudins (W0W averaging 0.38; Fig. 11). progressive deformation are: (1) the development of domino Evidence of a progression from symmetric to asymmetric boudins from a near-orthogonal crack in a layer that boudinage components, such as torn boudins with a later develops further by rotation and/or some internal defor- domino boudin component (Swanson, 1992) or shearband mation, e.g. that associated with the development of boudin component (‘intermediate’ boudinage of Lacassin flanking folds (Goscombe and Passchier, 2003); (2) (1988); Fig. 11; Appendix B) is also common. In a few forked-gash and sigmoidal-gash boudins initiate as orthog- sequential boudins, a third component can be recognized, onal cracks and develop by boudin separation and the inter- such as domino boudinage of a previously thinned, bow-tie boudin vein can either curve (sigmoidal-gash) or branch vein boudin (Fig. 11). The opposite, where dilational (forked-gash) at its tips (Goscombe and Passchier, 2003); domino boudins undergo symmetric necking of the inter- and (3) concave-face boudins have variably developed boudin vein material, has been modelled in progressive ‘burring’ over the boudin edge resulting in barrel boudins boudinage by Hossein (1970). In the high metamorphic and, with increasing strain, fishmouth boudins and false- grade areas investigated in the Kaoko Belt, the progression isoclinal folds. This is due to the maximum stress being from asymmetric to symmetric boudinage components is concentrated on the boudin edges, resulting in internal common. Domino boudins commonly experienced later deformation of the boudin block (Lloyd et al., 1982). extreme ductile stretch—either symmetric (tapering bou- More typically, sequential boudin structures record dinage) or slightly asymmetric (shearband boudinage)— components of different end-member boudin geometries, resulting in trains of widely isolated rhomb-shaped boudin superimposed in a temporal sequence (Table 2; Fig. 11). blocks (Fig. 11). We have called these ‘asymmetric- Sequential boudins are a special case of modified boudins, tapering’ boudins. In all investigated cases, the vergence involving progressive stretch of the layer, resulting in what of Sib inclination with respect to bulk shear sense and the can be considered boudinage-modified boudins. Eleven high u angles preserved are consistent with their initial percent of all investigated boudins are sequential boudins. formation as domino boudins (Fig. 12c). Asymmetric- In all examples investigated, there is no evidence for tapering sequential boudins are apparently restricted to high sequential boudins being the result of reworking by strain, high metamorphic grade ductile terranes. subsequent deformational events. All boudin components maintain the same extension axis (Le) and are consistent with progressive stretch in a single deformation event. 8. Multiple boudinage events Considering the four most common end-member boudin geometry types (drawn, shearband, domino and torn), there In all the multiply deformed terranes investigated are 12 possible permutations of sequential boudins invol- (Kaoko Belt, Ugab Terrane, Zambezi Belt and Adelaidean ving two different boudin components. Nine of these have Fold Belt), it has been found that boudinage occurred in been recorded in our dataset of natural boudin structures; more than one deformation event. In these terranes, the some of these are documented below and their frequency is vast majority of boudins formed early (88% in the Kaoko presented in Appendix B (this can be found in the online Belt), during a deformation event that produced a version of this paper). regionally pervasive penetrative fabric, and a minor The most common sequence is from a torn boudin group of boudins with different orientation formed in a component to a drawn boudin component by symmetric later deformation event. No cases were recorded of B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 759 boudinage from two distinct deformation events being following kinematic conclusions are generally applicable as superimposed to produce one complex boudin structure. they are based on a comprehensive set of observations and Given the possibility of multiple boudinage events, where data, from microscopic to macroscopic scales and a wide boudins are employed as shear sense indicators, flow range of geological settings, flow regimes, rock types and regime indicators, or indicators of the maximum exten- metamorphic grades. sion direction (Goscombe and Passchier, 2003), each Previous work has resulted in a number of kinematic boudin structure must be correctly correlated with the conclusions with regard to boudins and flow, and these are appropriate deformation event. dependent on the classification scheme outlined here. (1) Correlation of boudin structure and deformation event is Given that both the boudin block geometry and an possible because Le, the theoretical stretching axis associ- approximation of the fabric attractor can be identified, it ated with boudinage (normal to Lb within Se; Appendix A), has been shown that all boudin geometries in foliation- typically has a regular orientation and is both sub-parallel to oblique boudin trains and asymmetric boudins in foliation- and symmetrically distributed around the stretching linea- parallel boudin trains (Table 2) can be employed as practical tion (Lp) produced during the same deformation event and reliable shear sense indicators (Goscombe and Passch- (Goscombe and Passchier, 2003). The angle between Lp and ier, 2003). (2) It is widely recognized that the near Lb (d;Appendix A) has maximum frequency at 908 coincidence of the boudin extension axis (Le) with a (Goscombe and Passchier, 2003). Consequently, Le is a mineral stretching lineation (Lp)inmanyhostrocks useful kinematic indicator approximating the axis of suggests boudin structures can be used to constrain the maximum finite stretch. Where there are discrete orientation stretching axes (transport direction in monoclinic flow) in populations of Lb (and by inference also Le), these could be orogens devoid of mineral stretching lineations, such as in used to define multiple boudinage events. Where boudins fine-grained, low-strain or low metamorphic-grade terranes are developed, L may be the only kinematic indicator e (Goscombe and Passchier, 2003). available to define the stretching axis in low-grade, low- An important result of the work shown here is that strain or fine-grained terranes that are otherwise devoid of multiple generations of boudins, sequential boudin stretching lineations. Conversely, in terranes with mineral structures and reworked boudin structures, all record the lineations developed, boudin structures can be employed to strain history of a rock mass. Unlike deformation fabrics, test if these mineral lineations are indeed stretching boudins are difficult to completely rework and obliterate; lineations. thus multiple boudin generations can preserve palaeo- stretching axis directions from different deformational 9. Conclusions episodes. The suite of boudin types developed during different deformational episodes, and indeed between From this analysis of natural boudin structures, it can be different study areas, is often distinct. If future workers concluded that all simple unmodified boudin structures can draw correlations between boudin-type and flow regime, be fully classified by considering boudin block geometry, as seems to be the case, boudins may prove useful as boudin train obliquity and material layeredness as summar- indicators of flow regime during different deformational ized graphically in Appendix C (this can be found in the episodes and in characterizing different terranes (e.g. online version of this paper). The entire range of natural Passchier and Druguet, 2002). boudin block geometries can be rationalized in terms of only five distinctive end-members: drawn, torn, gash, domino and shearband boudins (Fig. 3). The process of boudinage Acknowledgements takes place in each case by only one of three mutually exclusive kinematic classes: no-slip, antithetic slip (A-slip) and synthetic slip boudinage (S-slip). Each of these three Drs Roy Miller, Rudolph Trouw, Fabio Pentagna, kinematic classes corresponds almost exclusively to specific Andre Ribeiro, Chris Wilson, Thomas Bekker, Dave Gray boudin block geometries (Table 1). All drawn and torn and Pat James as well as David and Audrey Goscombe, boudins form by no-slip boudinage, gash and domino Helmut Garoeb, Zigi Baugartner, Pete and Alex Siegfried, boudins by A-slip boudinage and shearband boudins by S- Murray Haseler and Bonza, are all thanked for their slip boudinage (Goscombe and Passchier, 2003). Tran- discussions, efforts spying out elusive boudins and great sitions between end-member boudin block geometries are company in the field. The constructive reviews and efforts recognized in individual geometric parameters (Figs. 2 and of Dr L. Goodwin and J. Hippertt were greatly 6–8; Table 2). Nevertheless, when the full suite of appreciated. This study resulted from regional mapping quantitative geometric parameters and qualitative features work undertaken for the Namibian Geological Survey and are considered, the end-member boudin block geometries self-funded fieldwork elsewhere. Mimi Duneski is sin- remain distinct groupings that are recognizable in the field. cerely thanked for her considerable administrative support We feel that the classification scheme outlined and the andhumourinNamibia. 760 B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763

Appendix A. Definition of boudin structural elements Boudin edge Intersection of a planar boudin face and geometric parameters (Fig. 1) (Sib) and boudin exterior (Sb); parallel to the long axis of both boudin blocks Regional fabric elements (Lb) and inter-boudin zones. Sp Penetrative foliation formed in host Lb Long axis of boudin; equivalent to the rock during deformation associated “neck line” (Hobbs et al. 1976). This with boudinage. line is usually parallel to the intermedi- Lp Stretching lineation in host rock, for ate axis (Y) of the finite strain ellipsoid. simplicity, considered vector of relative Equivalent to intersection between Sib transport during deformation associated and Sb surfaces, that is, the boudin edge with boudinage. Reference frame for (Fig. 1). linear boudin parameters. Lb vergence Vergence of acute angle between Lb and Se Enveloping surface of boudinaged layer, stretching lineation (Lp); either clock- equivalent to the overall trace of the wise or anticlockwise from the stretch- boudin train. In most cases parallel to Sp ing lineation in the Se surface. (i.e. foliation-parallel boudin trains) but Sb Boudin exterior; original surface of can be oblique to Sp in the case of boudin block in contact with host. foliation-oblique boudin trains. Refer- Block rotation Sense of relative rotation of individual ence frame for planar boudin parameters boudin blocks, with respect to bulk Le Extension axis defined by boudin struc- shear sense. ture. Line normal to the long axis of Boudin face Lateral end of boudin block adjacent to boudin (Lb),inSe plane. It is usually others in boudin train, typically a planar close to the X-axis of the finite strain or termination face (equivalent to Sib)or ellipsoid and typically sub-parallel to a point termination in drawn boudins the stretching lineation (Lp)when Sib Inter-boudin surface; defined as the developed. median plane in the inter-boudin zone, Shear sense Bulk shear sense during deformation at moderate to high angle to Se.In associated with boudinage. Reference asymmetric boudin structures, is typi- frame for all rotational and movement cally a single discrete surface that parameters in boudin structures. experienced lateral displacement of 0 Defined by C - and C-type shear band adjacent boudins. The Sib surface does cleavages (Simpson, 1984; Lister and not propagate beyond the boudinaged Snoke, 1984), s-type and d-type por- layer (in contrast to faults or shear phyroclasts (Passchier and Simpson, zones) and in asymmetric boudins, arcs 1986) and flanking folds (Hudleston, into the enveloping surface at the 1989; Passchier, 2001). boudin edge. In torn boudins, Sib is Profile plane Plane normal to both long axis of typically equivalent to the terminal face boudin (Lb)andSe. Boudins with of the boudin block. No Sib is developed monoclinic symmetry are viewed and in drawn boudins, thus an arbitrary parameters are measured in this plane. “Sib” is defined as the plane connecting In cases of triclinic boudin symmetry, boudin terminations, parallel and the Se surface is also used as an median to the extensional mobile zone additional, orthogonal plane of view. that is the neck (Fig. 4c). Fabric attractor Hypothetical plane or line in rock mass, Lib Vector of movement within Sib, rarely towards which material lines and planes expressed as slickenlines or mineral rotate with progressive deformation elongation lineations. (Passchier and Trouw, 1996; Passchier, Slip sense Sense of lateral displacement along Sib, 1997). with respect to bulk shear sense. Boudin nomenclature and structural elements Sib vergence There are two types of vergence of Sib Boudin block Separate elongate body of rock that with respect to bulk shear sense: (1) originated from boudinage of a layer, Spiral vergence, analogous to sygmoi- object or foliation planes. A string of dal tension gashes, and (2) Direction of boudin blocks originating from one inclination of Sib with respect to bulk layer comprise a boudin train. shear sense; those inclined in opposite Inter-boudin zone Elongate zone between the edges of direction to shear sense are named adjacent boudin blocks. forward-vergent and those inclined in B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 761

the same direction as shear sense are layer, parallel to Se and normal to Lb. named backward-vergent. These are Measured between boudin edges in equivalent to “forward-tilted” and asymmetric and torn boudins and “backward-tilted” respectively (Swan- between inflection points (Penge, son 1992), which are defined by an 1976) in the termination or neck zone imagined tilting of Sib from an orig- of drawn boudins (Fig. 1d). inally orthogonal orientation, with M0 Degree of isolation of adjacent boudin respect to bulk shear sense. In the case blocks, parallel to Se and normal to Lb. of gash boudins, both types of vergence Equivalent to “gap length” (Lloyd et al., 0 are observed on both the profile and Se 1982). M , M in asymmetric boudins planes of view. and M0 ¼ M is only possible in sym- “Drag” on Sib Sense of apparent drag on boudin face, metric boudins. with respect to slip sense. Can be either N Amount of dilation normal to, and antithetic (flanking folds) or synthetic across, Sib. Not applicable to drawn (flanking shear band; Fig. 5; Passchier, boudins with arbitrary “Sib”, minor for 2001). Flanking folds on Sib contribute asymmetric boudins and significant in an additional component of block torn boudins where with N ø M ø M0. rotation that is synthetic with whole D Lateral displacement along Sib between block rotation. Flanking folds on Sib are adjacent boudin blocks. not to be confused with the flanking Stretch Percent extension of the enveloping folds that formed by similar mechan- surface (Se): Stretch ¼ 100(M þ ism, but of different scale and site, on Lcosa)/L. Considers the extension of a the margin of the trace of foliation- line (Se), not a layer of finite width, thus oblique boudin trains (Fig. 5). stretch can only be considered mini- Angular parameters mum value of layer extension and d Acute angle between Lp and Lb,an stretch ,100% can occur for low M approximate measure of the degree of and high a boudins. departure from monoclinic symmetry. Bb Ratio describing the curve of the boudin A boudin structure of monoclinic sym- exterior (Fig. 4). Defined by the height metry has d = 908. above (þve) or below (2ve) the line a Relative block rotation; is the acute between boudin edges, divided by angle between Se and Sb. length of the straight line between u Acute angle between Sb and Sib; edges (i.e. L). quantifying boudin block shape. Bf Ratio describing the curve of the boudin 0 u Acute angle between Sib and Se; quanti- face (Fig. 4). Defined by the distance fying Sib inclination in outcrop: out (þve) or into the boudin block u0 ¼ u 2 a. (2ve) from the line between boudin b Angle between Sib and axial surface of edges, divided by the straight line associated kinkbands. Measured from length of this face line. Sib synthetic with respect to slip sense on Sib, and so may be acute or obtuse. C Obliquity of boudin train; measured as the angle between Se and the fabric References attractor, which is assumed in most cases to approximate the penetrative Aerden, D.G.A.M., 1991. Foliation-boudinage control on the formation of foliation (Sp). the Rosebery Pb–Zn orebody, Tasmania. Journal of Structural Geology Dimensional parameters 13, 759–775. W Width or thickness of boudinaged layer. Blumenfeld, P., 1983. Le “tuilage des megacristaux”, un crite`re d’ećoule- 0 ment rotationnel pour les fluidalites des roches magmatiques; W Minimum width of thinned layer or vein Application au granite de Barbey. Bulletin de la Socie´te´ geólogique infill in the inter-boudin zone. de France 25, 309–318. L Length of boudin blocks in the profile Borradaile, G.J., Bayly, M.B., Powell, C.McA., 1982. Atlas of Deforma- plane. tional and Metamorphic Rocks Fabrics, Springer-Verlag, Berlin, Q Extent of the boudin block parallel to Heidelberg, New York, 649pp. Bosworth, W., 1984. The relative roles of boudinage and ‘structural slicing’ Lb. Typically greater than the extent of in the disruption of layered rock sequences. Journal of Geology 92, the outcrop and thus effectively infinite. 447–456. M Component of extension of boudinaged Carreras, J., Druguet, E., 1994. Structural zonation as a result of 762 B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763

inhomogeneous non-coaxial deformation and its control on syntectonic Hudleston, P.J., 1989. The association of folds and veins in shear zones. intrusions: an example from the Cap de Creus area, eastern-Pyrenees. Journal of Structural Geology 11, 949–957. Journal of Structural Geology 16, 1525–1534. Jones, A.G., 1959. Vernon map-area British Columbia. Memoir Geological Casey, M., Dietrich, D., Ramsay, J.G., 1983. Methods for determining Survey Branch, Canada 296, 1–186. deformation history for chocolate tablet boudinage with fibrous Kraus, J., Williams, P.F., 1998. Relationships between foliation develop- crystals. Tectonophysics 92, 211–239. ment, porphyroblast growth and large-scale folding in the metaturbidite Conti, P., Funedda, A., 1998. Mylonite development in the Hercynian suite, Snow Lake, Canada. Journal of Structural Geology 20, 61–76. basement of Sardinia (Italy). Journal of Structural Geology 20, Lacassin, R., 1988. Large-scale foliation boudinage in gneisses. Journal of 121–133. Structural Geology 10, 643–647. Daniel, J.-M., Jolivet, L., Goffe, B., Poinssot, C., 1996. Crustal-scale strain Laubach, S.E., Reynolds, S.J., Spencer, J.E., 1989. Progressive deformation partitioning: footwall deformation below the Alpine Oligo–Miocene and superposed fabrics related to Cretaceous crustal underthrusting in detachment of Corsica. Journal of Structural Geology 18, 41–59. western Arizona, USA. Journal of Structural Geology 11, 735–749. Davis, G.H., 1987. A shear-zone model for the structural evolution of Lisle, R., 1996. Cover photograph. Journal of Structural Geology 18. metamorphic core complexes in southeastern Arizona. Continental Lister, G.S., Snoke, A.W., 1984. S–C mylonites. Journal of Structural Extensional Tectonics, Geological Society Special Publication 28, Geology 6, 617–638. 247–266. Lloyd, G.E., Ferguson, C.C., 1981. Boudinage structure: some new DePaor, D.G., Simpson, C., Bailey, C.M., McCaffrey, K.J.W., Bean, E., interpretations based on elastic–plastic finite element simulations. Gower, R.J.W., Aziz, G., 1991. The role of solution in the formation of Journal of Structural Geology 3, 117–128. boudinage and transverse veins in carbonate rocks at Rheems, Lloyd, G.E., Ferguson, C.C., Reading, K., 1982. A stress-transfer model for Pennsylvania. Geological Society of America Bulletin 103, the development of extension fracture boudinage. Journal of Structural 1552–1563. Geology 4, 355–372. Etchecopar, A., 1974. Simulation par ordinateur de la deformation Lohest, M., 1909. L’origine des veines et des geódes des terrains primaires progressive d’un aggregat polycrystallin. E´tude du de´veloppement de de Belgique. Annales de la Socie´te´ geólogique Belgique 36B, 275–282. structures orienteés par ećrasement et cisaillement. Unpublished Ph.D. Malavielle, J., Lacassin, R., 1988. ‘Bone-shaped’ boudins in progressive thesis, University of Nantes, France. shearing. Journal of Structural Geology 10, 335–345. Etchecopar, A., 1977. A plane kinematic model of progressive deformation Mandal, N., Karmakar, S., 1989. Boudinage in homogeneous foliation in a polycrystalline aggregate. Tectonophysics 39, 121–139. rocks. Tectonophysics 170, 151–158. Mullenax, A.C., Gray, D.R., 1984. Interaction of bed-parallel stylolites and Fry, N., 1984. The Field Description of Metamorphic Rocks, Geological extension veins in boudinage. Journal of Structural Geology 6, 63–71. Society of London Handbook Series, Open University Press, Milton Passchier, C.W., 1997. The fabric attractor. Journal of Structural Geology Keynes, 110pp. 19, 113–127. Ghosh, S.K., 1988. Theory of chocolate tablet boudinage. Journal of Passchier, C.W., 2001. Flanking structures. Journal of Structural Geology Structural Geology 10, 541–553. 23, 951–962. Ghosh, S.K., Sengupta, S., 1999. Boudinage and composite boudinage in Passchier, C.W., Druguet, E., 2002. Numerical modelling of asymmetric superimposed deformations and syntectonic migmatization. Journal of boudinage. Journal of Structural Geology 24, 1789–1804. Structural Geology 21, 97–110. Passchier, C.W., Simpson, C., 1986. Porphyroclast systems as kinematic Goldstein, A.G., 1988. Factors affecting the kinematic interpretation of indicators. Journal of Structural Geology 8, 831–843. asymmetric boudinage in shear zones. Journal of Structural Geology 10, Passchier, C.W., Myers, J.S., Kro¨ner, A., 1990. Field Geology of High- 707–715. Grade Gneiss Terrains, Springer-Verlag, Berlin, 150pp. Goscombe, B., 1991. Intense non-coaxial shear and the development of Penge, J., 1976. Experimental deformation of pinch-and-swell structures. mega-scale sheath folds in the Arunta Block, Central Australia. Journal Unpublished M.Sc. thesis, Imperial College, University of London. of Structural Geology 13, 299–318. Price, N.J., Cosgrove, J.W., 1990. Analysis of Geological Structures, Goscombe, B., 1992. High-grade reworking of central Australian Cambridge University Press, Cambridge. granulites. Part 1. Structural evolution. Tectonophysics 204, 361–399. Ramberg, H., 1952. The Origin of Metamorphic and Metasomatic Rocks, Goscombe, B., Everard, J.L., 2000. Tectonic evolution of Macquarie University of Chicago Press, Chicago. Island: extensional structures and block rotations in oceanic crust. Ramsay, A.C., 1881. The Geology of North Wales. Memoirs of the Journal of Structural Geology 23, 639–673. Geological Survey of Great Britain, 3. Goscombe, B., Passchier, C.W., 2003. Asymmetric boudins as shear sense Ramsay, J.G., 1967. Folding and Fracturing of Rocks, McGraw-Hill, New indicators—an assessment from field data. Journal of Structural York, pp. 103–109. Geology 25, 575–589. Ramsay, J.G., Huber, M.I., 1983. The Techniques of Modern Structural Grasemann, B., Stuwe, K., 2001. The development of flanking folds during Geology. Volume 1: Strain Analysis, Academic Press, London, pp. simple shear and their use as kinematic indicators. Journal of Structural 1–224. Geology 23, 715–724. Ramsay, J.G., Huber, M.I., 1987. The Techniques of Modern Structural Hanmer, S.K., 1984. The potential use of planar and elliptical structures as Geology. Volume 2: Folds and Fractures, Academic Press, London, pp. indicators of strain regime and kinemetics of tectonic flow. Current 516–633. Research, Part B, Geological Survey of Canada, Paper 84-1B, pp. 133– Ramsay, J.G., Lisle, R., 2000. The Techniques of Modern Structural 142. Geology. Volume 3: Application of Continuum Mechanics in Structural Hanmer, S., 1986. Asymmetrical pull-aparts and foliation fish as kinematic Geology, Academic Press, London. indicators. Journal of Structural Geology 8, 111–122. Roig, J.-Y., Faure, M., Truffert, C., 1998. Folding and granite emplacement Hanmer, S., 1989. Initiation of cataclastic flow in a mylonite zone. Journal inferred from structural, strain, TEM and gravimetric analyses: the case of Structural Geology 11, 751–762. study of the Tulle antiform, SW French Massif Central. Journal of Hanmer, S., Passchier, C., 1991. Shear-Sense Indicators: A Review. Structural Geology 20, 1169–1189. Geological Survey of Canada. Paper 90-17, 72pp. Sengupta, S., 1983. Folding of boudinaged layers. Journal of Structural Hobbs, B.E., Means, W.D., Williams, P.F., 1976. An Outline of Structural Geology 5, 197–210. Geology. John Wiley and Sons, 278–280. Simpson, C., 1984. Borrego Springs–Santa Rosa mylonite zone: a Late Hossein, K.M., 1970. Experimental work on boudinage. Unpublished M.Sc. Cretaceous west-directed thrust in southern California. Geology 12, 8–11. thesis, Imperial College, University of London. Simpson, C., Schmid, S.M., 1983. An evaluation of criteria to deduce the B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 763

sense of movement in sheared rocks. Bulletin of the Geological Society Sondre Stromfjord Airport, West Greenland. Tectonophysics 115, of America 94, 1281–1288. 297–313. Swanson, M.T., 1992. Late Acadian–Alleghenian transpressional defor- Weiss, L.E., 1972. The Minor Structures of Deformed Rocks, Springer- mation: evidence from asymmetric boudinage in the Casco Bay area, Verlag, Berlin, 431pp. coastal Maine. Journal of Structural Geology 14, 323–341. Wilson, G., 1961. The tectonic signiﬁcance of small-scale structures and Swanson, M.T., 1999. Kinematic indicators for regional dextral shear along their importance to the geologist in the ﬁeld. Annals of the Society of the Norumbega fault system in the Casco Bay area, coastal Maine. Geologists de Belgique 84, 424–548. Geological Society of America Special Paper 331, 1–23. Yamamoto, H., 1994. Kinematics of mylonitic rocks along the Median Van der Molen, I., 1985. Interlayer material transport during layer-normal Tectonic Line, Akaishi Range, central Japan. Journal of Structural shortening. Part II. Boudinage, Pinch-and-swell and migmatite at Geology 16, 61–70.